The best answer is D, Genghis Khan united the Mongols and built an empire by combining a superior cavalry with a confederation of tribes and clans. Ghengis Khan's calgary was light and fast moving and able to operate effectively at range and benefitted from more effective armament than its enemies, particularly in its use of a highly accurate compound bow. More importantly was Genghis Khan's ability to unite the various turkic tribes that made up the Mongol confederation into a coherent state capable of engaging in organized military campaigns.
